Angular(1) factory to create metronome based on Web Audio API with help from https://github.com/cwilso/metronome/

metronome-fct.js

'use strict';

/**
 * make a new Metronome based on the provided temp
 * ideas from:
 * https://github.com/cwilso/metronome/
 */
angular.module('main')
  .factory('Metronome', function ( $window ) {

    // build the metronome for injection
    var Metronome = function ( bpm ) {
      // boot the audio platform
      var AudioContext = $window.AudioContext || $window.webkitAudioContext;
      var audioContext = new AudioContext();
      audioContext.suspend();

      // decrease the volume and connect to output
      var gainNode = audioContext.createGain();
      gainNode.connect(audioContext.destination);
      gainNode.gain.value = 0.1;

      // specify the state
      var nextNoteTime = 0,
        currentSixteenth = 0,
        lookahead = 250,
        noteRes = 2, // 0 == 16th, 1 == 8th, 2 == 4th
        noteLen = 0.05,
        scheduleAheadTime = 0.25,
        timerID = 0;
      var isPlaying = false;

      // calculate actual length of 1/4 note
      // bpm is pulled from the song info, it won't change
      var secPerBeat = 60.0 / bpm;

      /**
       * advance note and playhead by a 16th note
       */
      var nextNote = function () {
        // add length of 16th note
        nextNoteTime += 0.25 * secPerBeat;
        // advance the 16th note marker
        currentSixteenth++;
        // wrap back to zero
        if ( currentSixteenth === 16 ) {
          currentSixteenth = 0;
        } // end if
      };

      /**
       * do the next note/pitch
       * @param beatNumber
       * @param time
       */
      var scheduleNote = function ( beatNumber, time ) {
        // we are skipping non-eighth sixteen notes
        if ( (noteRes === 1) && (beatNumber % 2) ) {
          return;
        } // end if

        // we are skipping non-quarter eighth notes
        if ( (noteRes === 2) && (beatNumber % 4) ) {
          return;
        } // end if

        // create osc
        var osc = audioContext.createOscillator();
        osc.connect(gainNode);

        // set some pitches to differentiate notes
        if ( !(beatNumber % 16) ) {
          osc.frequency.value = 1320.0; // Downbeat
        } else if ( beatNumber % 4 ) {
          osc.frequency.value = 440.0;  // Sixteenth Notes
        } else {
          osc.frequency.value = 880.0;  // Quarter Notes
        } // end if

        osc.start(time);
        osc.stop(time + noteLen);
      };

      /**
       * notes that need to play before the next interval
       * schedule these and advance playhead
       */
      var scheduler = function () {
        while ( nextNoteTime < (audioContext.currentTime + scheduleAheadTime) ) {
          scheduleNote(currentSixteenth, nextNoteTime);
          nextNote();
        } // end while
        timerID = setTimeout(scheduler, lookahead);
      };

      /**
       * start the metronome
       */
      this.play = function () {
        isPlaying = !isPlaying;

        if ( isPlaying ) {
          audioContext.resume();
          currentSixteenth = 0;
          nextNoteTime = audioContext.currentTime;
          scheduler();
        } else {
          clearTimeout(timerID);
        } // end if
      };

      /**
       * tell the controller if the metronome is playing
       * @returns {boolean}
       */
      this.getIsPlaying = function () {
        return isPlaying;
      };

      /**
       * tell the controller the current playhead time
       * @returns {*|number|Number}
       */
      this.getCurrentTime = function () {
        return audioContext.currentTime;
      };

    };

    return Metronome;

  });